Optical networks are becoming increasingly common because of the extremely wide bandwidth that can be supported by optical transmission techniques. Many, if not most, optical networks utilize wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) in order to maximize the amount of information that can be transported over the network per unit time (i.e., data bandwidth). Like all information networks, switching or routing devices are provided in the network to direct signals between nodes of the network to assure that information originated at a first node (e.g., a server node) and intended for a second node (e.g., a client node) is routed through the network from the first node to the intended second node. Switching and routing of signals on optical networks is commonly achieved using filters and optical routing components such as, fixed optical add-drop multiplexers (fixed OADMs), reconfigurable optical add/drop multiplexers (ROADMs), and/or optical cross-connects (OXCs). These types of routing devices, particularly ROADMs, are popular because they are extremely flexible in routing ability. However, they are relatively expensive because, among other reasons, they are relatively complex and incorporate active optical elements.
Furthermore, it is difficult to alter a network after it has been set up using such components. For instance, there are several standard wavelength grids in common use in DWDM optical networks, including 200 GHz, 100 GHz and 50 GHz grids. Each of these standards defines a grid of wavelengths for DWDM within a portion of the visible light spectrum (e.g., C band such as approximately 1525-1565 nm). For instance, the 200 GHz grid can define a grid of 22 wavelengths for DWDM at spacings of 200 GHz in C band, the 100 GHz grid can define a grid of 44 wavelengths for DWDM at spacings of 100 GHz in C band, and the 50 GHz grid can define a grid of 88 wavelengths for DWDM at spacings of 50 GHz in C band. An exemplary standard is ITU-T G.694.1, “Spectral grids for WDM applications: DWDM frequency grid,” May 2002, the contents of which are incorporated in full by reference herein. As technology improves, the wavelengths on which different data signals may be transported are likely to become increasingly densely packed. It is envisioned that wavelength density in DWDM optical networks will continue to increase and that practical networks soon will be able to be implemented with arbitrary grid spacing to enable richer-spectrum sources such as OFDM and so-called super-channels.
Fixed OADMs have a colored structure, wherein each port is associated with a particular wavelength. Therefore, to change the wavelengths used in an existing network built using fixed OADMs would potentially require replacement of some or all of the fixed OADMs in the network, which is an expensive proposition to the point of being impractical in many cases. ROADMs, on the other hand, can be reconfigured remotely to alter their wavelength characteristics to work with different wavelengths. However, ROADMs have a banded structure, meaning that, while each port can be reconfigured to any wavelength, the wavelength spacing is still fixed. Thus, a change in the grid spacing in a ROADM-based network would require replacement of all of the ROADMs. Furthermore, even if only the wavelengths, but not the spacings, are to be altered in a network built with ROADMs, very detailed planning is required. Even further, ROADMs employ a channel filter for each wavelength/port. These channel filters introduce loss and signal distortion, thus limiting the number of ROADMs that a signal may pass through before it is too attenuated and/or distorted to be adequately detected at a receiver.
Ciena Corporation through Nortel Networks has an optical networking platform that relies on coherent detection of specific wavelengths in which receivers on the network are able to tune into particular frequencies without the need for optical filters. Accordingly, a fiber in a DWDM network bearing different signals on different wavelengths can be coupled directly to a receiver employing coherent detection, and the receiver is able to pick out data on a particular wavelength without the need for a channel filter. For further explanation of coherent detection and, particularly, the coherent detection scheme developed by Ciena Corporation, reference can be had to an number of resources, such as Sun, H. et al, Real-time measurement of a 40 Gb/s coherent system, Optics Express, Vol. 16 No. 2, Jan. 21, 2008 and Nelson, L. E. et al., Performance of a 46-Gbps dual-polarization QPSK, Conference Paper, Optical Fiber Communication Conference (OFC), San Diego, Calif., Feb. 24, 2008.
Conventional networks rely on complex optical filtering devices to provide reconfigurability in the optical domain. Devices like Wavelength Selective Switches (WSSs) are commonplace in these solutions. Multiple WSS's are used in multi-degree ROADMS. In long-haul applications, there is a benefit to having WSSs in the ROADM application. The WSS can provide optical filtering and per-channel equalization. The optical filtering is important in mesh applications as it eliminates noise-funneling from multiple amplified lines, and allows the re-use of a wavelength in the mesh for multiple point to point demands which reduces a phenomenon often called wavelength exhaust. The equalization also provides a way to optimize the per-channel Optical Signal-to-Noise Ratio (OSNR), which ultimately reduces the number of Optical-Electrical-Optical (O-E-O) regenerators in a network deployment, and in the end saves cost. In metro applications, there are no opportunities to eliminate regeneration points, and so the performance benefit of the WSS is not needed. Also, in a limited size deployment the deleterious effect of noise funneling in a mesh network can be managed at an acceptable level. Therefore, the WSS is not necessary for performance reasons and a greatly reduced number of WSS's can be used to address the issue of wavelength exhaust when compared to using them for all add/drop locations.